Method and evaporator device for evaporating a low temperature liquid medium

ABSTRACT

In a method and evaporator device for evaporating a low temperature liquid medium, such as hydrogen for example, the hydrogen is first evaporated and at least partially superheated in a forward-flowing first channel, and is then directed to flow back in the opposite direction in a second return-flowing channel. The second channel is especially interposed between the first channel and a passage through which flows a heat-providing medium such as a hot exhaust gas. Thus, the superheated hydrogen flowing in the second channel serves as an intermediate layer for heat transfer from the heat-providing medium through the hydrogen in the second channel to the low temperature, initially liquid hydrogen in the first channel. The heat exchange surfaces in contact with the heat-providing medium are not directly adjacent the extremely cold surfaces in contact with the in-flowing low temperature liquid hydrogen, and the superheated hydrogen acts as a buffer between the hot side and the cold side of the evaporator. As a result, the evaporator has a very low structural weight, and condensation and icing problems can be avoided even when using a hot medium and a cold medium having extremely different temperatures.

FIELD OF THE INVENTION

The invention relates to a method and an evaporator device forevaporating a low temperature liquid medium such as hydrogen, using anevaporator operating as a heat exchanger between a hot medium that givesoff its heat and the liquid medium that is to be evaporated.

BACKGROUND INFORMATION

Evaporators of the above mentioned general type are typically embodiedas plate-type or tubular heat exchangers. Such heat exchangers aretypically used in applications in which a medium is stored in liquidform at a low or super-cold temperature in a storage tank, but is to beprovided for use in a gaseous physical phase. An example of such anapplication is the use of cryogenic liquids, such as liquid hydrogen orliquid natural gas, as a fuel or energy carrying medium for aircraftpropulsion engines, and especially turbine engines.

A particular feature relating to such evaporator devices is that theliquid medium to be evaporated is delivered to the evaporator with avery low inlet temperature of only about 20 K (-253° C.). On the otherhand, a relatively hot medium, such as the surrounding atmospheric air,or the exhaust gas of an engine for example, or some other heatedmedium, is provided to the evaporator as a heat source for evaporatingand in some cases superheating the cryogenic liquid medium. Thus, thehot medium comes into contact with extremely cold surfaces of theevaporator, which are cooled by the cryogenic medium that is to beevaporated. As a result there is a danger that the hot medium will becooled to below its respective dew point temperature or freezing pointtemperature so that it at least partially condenses or forms ice on thesurfaces of the evaporator. Such condensation or ice build-up obstructsthe flow passages for the hot medium, interferes with the operation ofthe evaporator, and can present a serious danger, especially if theevaporator is part of a fuel preparation system for an aircraft engine.

Typically, this danger of freezing or condensing of the hot medium isavoided or counter-acted by reducing the heat transfer between the hotmedium and the cryogenic medium that is to be evaporated. In thismanner, higher surface temperatures can be achieved at the heat inletside of the evaporator. However, this in turn necessitates a larger andheavier structure of such an evaporator in order to achieve the sametotal or overall heat transfer and evaporation of the cryogenic medium,which is especially undesirable in the application of such evaporatorsin aircraft and spacecraft.

OBJECTS OF THE INVENTION

In view of the above it is the aim of the invention achieve thefollowing objects singly or in combination:

to provide a method and an evaporator device for evaporating a lowtemperature liquid medium in such a manner so as to reliably avoid thecondensation or ice formation of the hot medium such as air or exhaustgases that are provided to the evaporator;

to provide such an evaporator device with a particularly compact andlightweight structure while still achieving an efficient heat transferand avoiding the formation of condensate or ice; and

to provide a construction of an evaporator device that can convenientlybe incorporated or integrated into a hot gas duct or even a combustionchamber of an engine such as a turbine engine.

SUMMARY OF THE INVENTION

The above objects have been achieved in a method of evaporating a liquidmedium such as hydrogen in an evaporator operating as a heat exchanger,according to the invention, wherein the hydrogen first flows through afirst portion of a flow channel where it is evaporated and superheatedto a certain extent, and then flows back through a second portion of theflow channel where it is further heated. The hydrogen in the secondchannel portion also acts as an intermediary or buffer for heat transferfrom the hot medium through the hydrogen in the second channel portionand to the hydrogen in the first channel portion. The net direction offlow of the hydrogen in the second channel portion is substantiallyopposite the direction of flow of the hydrogen in the first channelportion.

The above objects are further achieved in the evaporator deviceaccording to the invention, having a flow channel for the medium to beevaporated and a passage for the heat-providing medium. The channel isarranged outwardly around the passage and comprises an initial or firstportion and a second portion with a return flow in the oppositedirection from that of the first portion. To achieve this, the secondchannel portion is connected in series flow communication with adownstream end of the first channel portion. More particularly, thefirst portion and the second portion of the flow channel for the mediumto be evaporated are substantially cylindrical channels arrangedconcentrically outside of and around the passage through which theheat-providing medium flows.

Preferably, at least a part of the second return-flow portion of thefirst channel is arranged radially inwardly from the first initialportion of the channel, so that the second return flow portion isbetween the first portion of the channel and the hot medium passage. Thehot medium passage and the second channel portion thus share a commonboundary wall therebetween, i.e. the hot medium is on one side of thiscommon boundary wall while the colder medium is on the other side ofthis common boundary wall. Similarly, the first and second channelportions share an internal boundary wall therebetween, while the firstchannel portion is further bounded by an outer boundary wall oppositethe internal boundary wall, whereby this outer boundary wall may formthe outer wall of the overall apparatus. The hot medium does not flowanywhere directly along the outer boundary wall. In this manner, thesurfaces of the evaporator adjacent or bounding the hot medium passageare not directly exposed to the extreme low temperature liquid mediumentering the evaporator at the inlet thereof, but instead are onlyexposed to the medium in the return flow channel portion where themedium has already been evaporated and partially superheated.

According to further details of the invention, the first forward flowportion and the second return flow portion of the channel for the mediumto be evaporated can have a spiral or helical flow channel shape,wherein the spiral rotation direction can be the same or different inthe two flow channel portions. In a further embodiment, at least eitherthe forward flowing channel portion or the return flowing channelportion can comprise a helical or spiral coiled tube arranged within acylindrical space. In such an embodiment, the net flow direction of themedium refers to a substantially axial direction to the right or to theleft, while the incremental or local flow direction at any point in thehelical flow channels includes a substantial circular or circumferentialcomponent.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now bedescribed, by way of example, with reference to the accompanyingdrawings, wherein:

FIG. 1 is a lengthwise sectional view through an evaporator deviceaccording to a first embodiment of the invention;

FIG. 2 is a detail sectional view showing a portion of an evaporatorgenerally according to FIG. 1 with a variation of the first embodiment;

FIG. 3 is a lengthwise sectional view through an evaporator device usinga helical tube as a flow channel according to a second embodiment of theinvention; and

FIG. 4 is a detail sectional view showing a portion of an evaporatordevice generally according to FIG. 3, but with a variation of the secondembodiment.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BESTMODE OF THE INVENTION

FIG. 1 shows one possible embodiment of an evaporator 1 according to theinvention, which may be used for evaporating hydrogen as a fuel for anaircraft propulsion turbine engine, for example. The evaporator 1principally comprises three concentrically arranged pipes 2, 3, and 4,wherein a respective helical groove or channel 5 and 6 is provided onthe outer surfaces of the two radially inner pipes 3 and 4. Each of thegrooves 5 and 6 may be a single continuous helical groove, or maycomprise a respective plurality of parallel helical grooves. Forexample, two separate, interleaved grooves 5 may be provided in the pipe3.

The grooves or channels 5 and 6 may be formed in various ways. Forexample, the grooves 5 and 6 may be milled or otherwise machined intothe outer circumferential surfaces of the pipes 3 and 4 while leavingradially extending helical ribs or walls 5' and 6' between adjacentportions of the grooves. Alternatively, the grooves or channels 5 and 6may be machined or provided on the inner surfaces of the pipes 2 and 3,but the required machining operations are simpler in the preferredarrangement described above. As a further alternative to machining,separate ribs or webs 5' and 6' may be mounted, e.g. by welding, on thepipes 3 and 4 to form the grooves or channels 5 and 6 therebetween. Theribs or webs 5' and 6' forming the walls of the channels are furtherpreferably connected to the respective inner surfaces of the adjacentradially outer pipes.

The outer pipe 2 and the inner pipe 4 are respectively joined and sealedtogether in a gas-tight manner by two end walls 7 and 8. The middle pipe3 is also joined to the end wall 7 in a gas-tight manner, but a gap S isleft to remain between the right-hand end of the middle pipe 3 (as shownin FIG. 1) and the end wall 8. This gap S provides a flow communicationpassage or flow port from the first channel portion 5 to the secondchannel portion 6 to form a continuous flow channel.

The evaporator 1 further includes an inlet 10 communicating with anupstream end of the first channel portion 5, such that a liquid mediumto be evaporated, such as hydrogen, can be introduced in the directionof arrow 9 through the inlet 10 and into the first channel portion 5.Once the medium such as hydrogen has flowed through the first channelportion 5 and the second channel portion 6, the resulting gas exits fromthe evaporator 1 in the direction of arrow 12 through the outlet 11provided in communication with a downstream end of the second channelportion 6. A layer of any known insulation material 13 is arranged onthe outer surfaces of the evaporator 1 that are exposed to the very coldtemperatures of the medium to be evaporated, in order to avoidcondensation or icing on these surfaces. A heat-providing medium flowsin the direction of arrow 14 through a hot gas passage provided by theinner pipe 4, during operation of the evaporator 1.

With the above arrangement, the medium that is to be evaporated flowsthrough the inlet 10 and then into and through the first channel portion5 in a direction progressing from left to right in FIG. 1. The mediumthen flows through the gap S into the second channel portion 6, throughwhich the medium then flows in a reverse direction progressing fromright to left in FIG. 1 to finally exit from the outlet 11. Thus, themedium to be evaporated first flows through a forward-flowing channelportion 5, and then flows back in the opposite direction through areturn-flowing channel portion 6.

As the medium flows through the first channel portion 5 and the secondchannel portion 6, it is heated and thus evaporated and superheated bythermal energy provided by the hot medium flowing in the direction ofarrow 14 in the hot gas passage formed by the pipe 4. As represented byarrow H, the heat is transferred from the hot gas, radially outwardlyfirst to the medium in the second channel portion 6, which has alreadybeen evaporated and partially superheated, and then to the colder mediumin the first channel portion 5. Thus, the first channel portion 5provides a pre-heating and pre-evaporating channel, while the secondchannel portion 6 forms a super-heating and heat transfer intermediaryor buffer channel. With this arrangement, the outer surface of the pipe4 is only in contact with gaseous medium in the second channel portion 6that has already been evaporated and partially superheated, and is notdirectly in contact with the extremely cold liquid medium entering theinlet 10 and flowing in the first channel portion 5. The medium in thesecond channel portion 6 forms a buffer or intermediate layer betweenthe hot gas flowing along arrow 14, and the extremely cold liquid mediumflowing in the inlet 10 and the first channel portion 5. In this manner,condensation and icing of the hot gas flowing along the inner surface ofthe inner pipe 4 is avoided.

In the first embodiment of FIG. 1, the first channel portion 5 and thesecond channel portion 6 may have either the same or oppositely directedhelical rotations. In other words, both the first channel portion 5 andthe second channel portion 6 may have a clockwise rotation or acounterclockwise rotation, or one channel portion may have a clockwiserotation while the other channel portion has a counterclockwiserotation. On the other hand, the variation in FIG. 2 is only possiblewhen both helical channel portions 5 and 6 have the same direction ofrotation. In the variation according to FIG. 2, the walls 5' and 6' ofthe channel portions 5 and 6 are arranged in radial registration oralignment with one another. Such an arrangement achieves a particularhigh strength and rigidity of the evaporator 1, with a lightweightconstruction. Both the outer wall formed by the outer pipe 2 and theinner wall formed by the inner pipe 4 are well supported againstdeformation. However, this arrangement provides a more-direct thermalconduction path between the hot side and the cold side of theevaporator, which may be undesirable due to the above discussedcondensation and icing problems in particular situations.

FIG. 3 shows a second embodiment of the invention in the form of anevaporator 15 including an outer shell or casing 16, an inner shell orcasing 17, two end walls 18 and 19, an inlet 20, an outlet 21, and aninsulating layer 22. The outer casing 16 and the inner casing 17,together with the end walls 18 and 19, enclose a gas-tight hollow space23 substantially in the form of an annular space. In this evaporator 15,similarly to the above described evaporator 1, the flow path for themedium to be evaporated includes a first channel portion through whichthe medium will flow in a forward direction, and a second channelportion through which the medium will flow in a return direction.

In order to achieve this, a helical or spiral tube 24 is arranged in thehollow space 23 in such a manner so that the spiral tube 24 uniformlycontacts the outer casing 16 and the inner casing 17. Thus, the firstchannel portion is formed within the spiral tube 24, and the secondchannel portion is formed in the space 23 outside of the spiral tube 24.The left end of the spiral tube 24 as shown in FIG. 3 is connected onlyto the inlet 20, while the right end of the spiral tube 24 is open so asto communicate with the hollow space 23. Thus, if the medium to beevaporated is directed into the inlet 20, it then flows through thehelical tube 24 from the left to the right as shown in FIG. 3, and thenexits from the helical tube 24 into the hollow space 23. From here, themedium flows in the reverse direction, namely from the right to theleft, through the channel spaces formed between the tube 24 and theouter casing 16 and the inner casing 17 respectively, to ultimately flowout of the outlet 21. The channel spaces remaining within the hollowspace 23 have a substantial gusset wedge or hour-glass cross-section,depending upon how tightly together the spiral coils of the tube 24 arearranged, i. e. depending on whether or not the adjacent coils of thespiral tube 24 contact one another. A spiral tube may alternatively oradditionally be arranged to form the second return-flowing channelportion in a similar manner.

A hot medium flows in the direction of arrow 25 through a hot gaspassage provided by the inner casing 17, and transfers heat to themedium to be evaporated within the spiral tube 24 and the annular space23. As a result, the medium exits from the outlet 21 in a gaseousphysical state. The already-heated medium in the spaces 23 forms abuffer between the hot casing 17 and the cold liquid medium in thespiral tube 24, and especially near the inlet end thereof.

FIG. 4 shows a further variation of the embodiment of an evaporatorshown in FIG. 3. In this variation, a spiral metal web or rib 26 iswrapped around the outer surface of the inner casing 17, and connectedthereto, for example by welding. Thus, a web or rib 26 is providedbetween each adjacent pair of coils of the spiral tube 24. Thisarrangement is effective to strengthen and stiffen the inner casing 17of the evaporator in situations in which a high internal pressure isexpected within the evaporator. In this manner, denting or bending ofthe inner casing 17 can be avoided. Furthermore, this arrangement canespecially provide a plurality of parallel-connected flow channelsmaking up the second channel portion. However, the webs 26 provide apath of thermal conduction between the hot casing 16 and the cold spiraltube 24, which may be undesirable in some situations.

A method according to the invention can be carried out using any one ofthe evaporators shown in FIGS. 1 to 4, or other variants.

According to the method, the medium, which may be hydrogen for example,is first evaporated and at least partially superheated while it flowsthrough a first forward-flowing channel portion, and is then directed toflow back through a return-flowing channel portion along the surface ofthe forward-flowing channel portion. In the second channel portion, themedium is used as an intermediary layer for the heat transfer from theheat-providing medium to the medium that is to be evaporated in thefirst channel portion. In the second channel portion, the medium isfurther heated by heat being transferred from the heat-providing medium,but is also cooled by further transfer of the heat to the medium in thefirst channel portion. By appropriately dimensioning and configuring theevaporator, a re-condensing of the medium in the second channel portioncan be avoided, especially since the superheated medium within thesecond channel portion will only re-condense if it is again cooled downto its low dew point temperature of about 20 K.

The method and evaporator described herein, using a two-stage reversingflow evaporation, achieves advantages especially when a hot medium and acold medium of extremely different temperatures are to be handled, sinceicing and condensation can be avoided. Furthermore, the inventionsimultaneously achieves an extremely low structural weight.

In a practical application, the inner pipe or casing 4, 17 is acomponent of a hot gas duct of a turbine engine or the like.Alternatively, the present method and evaporator can also successfullybe used if the pipe 4 or inner casing 17 or other heat-providing elementis incorporated into a combustion chamber of a combustion engine, suchas a gas turbine engine.

Although the invention has been described with reference to specificexample embodiments, it will be appreciated that it is intended to coverall modifications and equivalents within the scope of the appendedclaims. The present disclosure also covers all possible combinations ofany of the various features recited in the several claims.

What is claimed is:
 1. An evaporator apparatus for evaporating a firstmedium that is initially in liquid form, through heat transfer from asecond medium, comprising:a hot medium passage adapted to have thesecond medium flow therethrough, and a channel adapted to have the firstmedium flow therethrough and be evaporated therein; wherein: saidchannel is arranged outwardly around said hot medium passage; saidchannel includes a first forward-flowing channel portion adapted to havethe first medium flow therein in a first net direction, and a secondreturn-flowing channel portion connected in series flow communication ata downstream end of said first channel portion and adapted to have thefirst medium flow therein in a second net direction substantiallyopposite said first net direction; at least a part of said secondchannel portion is arranged between the entirety of said hot mediumpassage and the entirety of said first channel portion; said hot mediumpassage, said first channel portion and said second channel portion areso arranged relative to each other so that said hot medium passage andsaid second channel portion share a common boundary wall therebetween,and so that said hot medium passage and said first channel portion donot share a common boundary wall anywhere in said evaporator apparatus;and said first channel portion comprises a first helical flow channel,said second channel portion comprises a second helical flow channel, andsaid first and second helical flow channels respectively have helicalrotations in opposite rotation directions.
 2. The evaporator apparatusin accordance with claim 1, further comprising an inlet for said firstmedium arranged at and connected for flow communication to an upstreamend of said first channel portion, an outlet for said first mediumarranged at and connected for flow communication from a downstream endof said second channel portion, and a flow port provided at saiddownstream end of said first channel portion and arranged for flowcommunication to an upstream end of said second channel portion, whereinsaid upstream end of said first channel portion is adjacent saiddownstream end of said second channel portion, and said downstream endof said first channel portion is adjacent said upstream end of saidsecond channel portion.
 3. The evaporator apparatus in accordance withclaim 1, wherein said channel is arranged concentrically and radiallyoutwardly around said hot medium passage, and wherein said at least apart of said second channel portion is arranged radially between saidhot medium passage and said first channel portion.
 4. The evaporatorapparatus in accordance with claim 3, wherein all of said second channelportion is arranged radially between said hot medium passage and saidfirst channel portion.
 5. The evaporator apparatus in accordance withclaim 1, wherein said hot medium passage and said channel respectivelycomprise and are bounded by concentrically arranged cylindrical walls,wherein at least one of said walls has grooves milled therein withradially extending webs remaining between adjacent portions of saidgrooves, wherein said webs are integral and unitary with said at leastone of said walls, so as to form at least one of said first channelportion and said second channel portion in said milled grooves.
 6. Theevaporator apparatus in accordance with claim 1, wherein said secondchannel portion comprises a plurality of flow channels connected andextending in parallel with one anther.
 7. The evaporator apparatus inaccordance with claim 1, wherein said hot medium passage is a combustionchamber.
 8. A method of using the evaporator apparatus according toclaim 1 for evaporating a first medium that is initially in liquid form,said method comprising the following steps:(a) introducing said firstmedium in liquid form into said first channel portion, and flowing saidfirst medium in a first net direction in said first channel portion, soas to evaporate and at least partially superheat said first medium; (b)after said step (a), flowing said first medium in a second net directionsubstantially opposite said first net direction in said second channelportion; and (c) flowing said second medium in said hot medium passage;wherein heat is transferred from said second medium into said firstmedium in said second channel portion, and heat is transferred from saidfirst medium in said second channel portion into said first medium insaid first channel portion, without transferring any heat directly fromsaid second medium into said first medium in said first channel portion.9. The method according to claim 8, wherein said step (b) comprisesflowing said first medium in said second channel portion so as to bephysically between all of said first medium in said first channelportion and all of said hot medium, and so as to flow along a hottersurface directly adjacent said hot medium passage and a colder surfacedirectly adjacent said first channel portion.
 10. The method accordingto claim 8, wherein said first medium in said second channel portionforms an intermediate heat transfer buffer between said hot medium andsaid first medium in said first channel portion.
 11. The methodaccording to claim 8, wherein said first medium is hydrogen that isinitially in cryogenic liquid form when it is introduced into said firstchannel portion.
 12. The method according to claim 8, wherein said hotmedium is a combustion exhaust gas from a combustion engine.
 13. Theevaporator apparatus in accordance with claim 1, wherein said firstchannel portion includes a radially outer boundary wall that forms anouter wall of said apparatus and that does not bound any portion of saidhot medium passage.
 14. The evaporator apparatus in accordance withclaim 13, further comprising a thermal insulation layer arrangedradially outwardly around said radially outer boundary wall.
 15. Theevaporator apparatus in accordance with claim 1, wherein said firstchannel portion and said second channel portion share an internalboundary wall therebetween, wherein said first channel portion furtherincludes an outer boundary wall opposite said internal boundary wall,and wherein said first channel portion and said second channel portionare so arranged relative to each other so that a heat transfer occursfrom the second medium in said hot medium passage to the first medium insaid second channel portion through said common boundary wall, and aheat transfer occurs from the first medium in the second channel portionto the first medium in the first channel portion through said internalboundary wall, and no heat transfer occurs directly from the secondmedium to the first medium in said first channel portion through saidouter boundary wall.
 16. The evaporator apparatus in accordance withclaim 1, wherein said hot medium passage in its entirety is a straightcylindrical passage extending from an inlet into said apparatus at oneend of said apparatus to an outlet from said apparatus at another end ofsaid apparatus, and said hot medium passage and said channel are soarranged that the second medium is always constrained to be radiallyinwardly located relative to said channel.
 17. A combination comprisingsaid evaporator apparatus according to claim 1, said second medium whichcomprises a combustion gas, and said first medium which comprises acryogenic liquid when it is introduced into said first channel portion.